PDA

View Full Version : 7199 failure? - Ampeg Gemini I


Tres Mellow
03-22-2010, 08:57 PM
The background:
My '65 Ampeg Gemini I was recapped (power stage) by a tech before I got it and I replaced all the tubular caps in the amp (most tested leaky/bad) and a couple small bias caps for the reverb and tremolo circuits. I replaced the Sovtek 7591XYZs it came with with JJ 7591s (no alterations had been made to the circuit for the 7591XYZs) and replaced all the 12ax7s with modern JJs. I replaced the original Ampeg 7199 with an NOS RCA blackplate 7199. Since then it has been very quiet at idle and has been sounding fantastic.

The problem:
Tonight, while the amp was warming up I heard a few faint pings. The first strum of the guitar was loud and clear immediately followed by a drastic weakening in the sound and crappy distorted sound quality--both channels, all 4 inputs, different guitar cable, guitar, etc. I turned the amp off, waited a bit and turned it on again. The same thing happened.

So for lack of a better idea, I put the Ampeg 7199 back in and played the amp for about 45 minutes and everything now sounds great again and works as it should.

Is this typical of what a 7199 sounds like when it fails? The suddenness surprised me, and I want to make sure it might not be something else that the replaced 7199 is masking.

Also, I understand that the NOS 7199s are far preferable to the more recently produced ones. What about converting the 7199 to 6GH8A or 6U8A?

Thanks:aok

Blue Strat
03-22-2010, 09:17 PM
There's no particular "typical" sound a 7199 makes when failing.

Try cleaning the pins on the RCA 7199, or even just try it again before condemning that tube. Check the FAQ on my website, under Resources, about cleaning tube pins and sockets.

hasserl
03-22-2010, 09:37 PM
Off topic, I'd just like to say to Blue Strat, nice to have you back active around here. You were missed.

Ronsonic
03-22-2010, 11:45 PM
The conversion to 6U8 etc. requires moving three wires on the socket. The recent unavailability of 7199 has hit my bench pretty hard. The good news is I've got quantities of NASA pulled 6U8A from a surplus dealer here in Florida. Generally in audio amps the pentode section is used for gain with the triode as a phase splitter so the mu difference doesn't affect anything. Can't guarantee it can't be heard, but it sounds way better than a broke amp.

Oh, and listen to Mike.

Blue Strat
03-23-2010, 05:53 AM
Off topic, I'd just like to say to Blue Strat, nice to have you back active around here. You were missed.

Thanks dude! Glad to be back. :JAM

Tres Mellow
03-23-2010, 06:16 AM
Thanks everybody. I'll try cleaning the pins, Mike. It certainly makes sense that just changing or reseating the 7199 could be enough to clear up the issue if the sockets are oxidized. I was under the impression that 7199s are very long-lasting tubes, so the suddenness of the sound drop-out surprised me.

Blue Strat
03-23-2010, 06:25 AM
Thanks everybody. I'll try cleaning the pins, Mike. It certainly makes sense that just changing or reseating the 7199 could be enough to clear up the issue if the sockets are oxidized. I was under the impression that 7199s are very long-lasting tubes, so the suddenness of the sound drop-out surprised me.

Anything can and will fail at any time. Tubes aren't bullet proof and testing doesn't predict the future. Don't assume anything. (can't think of any more platitudes before my second cup of coffee). ;)

mark norwine
03-23-2010, 06:38 AM
I'm going to toss this out there.....and I fully expect "rolled eyes", but I swear it's true.

I have a friend with a RRII which also has the 7199 in that position. He said "it makes weird noises".

That amp was on my bench about 5 times.....I could NOT make it make noise. But each time my friend got it home, noise would return

Rinse & repeat.

On my bench, I pulled the chassis, took out the tubes, cleaned everything.....you name it. Could not make it fail. Each time, I'd button it up & send it out the door.

.....and back it would come.

One day, my friend called me. It made noises at a gig (again) and he tried to "field fix" out of desperation. He took the metal tube shield off of the 7199.........





(wait for it!)





....and the noises ceased. Never came back.

He's been problem free for years now....no shield on the 7199

Go figure.

Blue Strat
03-23-2010, 08:00 AM
I'm going to toss this out there.....and I fully expect "rolled eyes", but I swear it's true.

I have a friend with a RRII which also has the 7199 in that position. He said "it makes weird noises".

That amp was on my bench about 5 times.....I could NOT make it make noise. But each time my friend got it home, noise would return

Rinse & repeat.

On my bench, I pulled the chassis, took out the tubes, cleaned everything.....you name it. Could not make it fail. Each time, I'd button it up & send it out the door.

.....and back it would come.

One day, my friend called me. It made noises at a gig (again) and he tried to "field fix" out of desperation. He took the metal tube shield off of the 7199.........





(wait for it!)





....and the noises ceased. Never came back.

He's been problem free for years now....no shield on the 7199

Go figure.

Pentode? Heat? Who knows.

mark norwine
03-23-2010, 08:04 AM
I'm a decent tech I suppose, but I'll also be the first to say that I have no way of *really* knowing what's happening on a sub-atomic basis within the tube.

donnyjaguar
03-23-2010, 08:43 AM
The 7199 can easily be modded out with more common (cheaper) tubes like the 6U8 and 6GH8A. The latter will call for changes to the plate and cathode resistors typically, but in my opinion isn't detrimental to performance.

Tres Mellow
03-23-2010, 09:11 AM
I see Antique Electronics Supply has a socket adapter for 7199 to 6GH8A. Any reason not to use them so the tube socket doesn't have to be re-wired (in case 7199s go into production again), other than applications where you can't spare and extra .64" clearance?

mark norwine
03-23-2010, 09:41 AM
Personally, I've never been a fan of "adaptors"; if you want to use another tube, configure the socket for another tube.

I wouldn't worry about 7199 going into production. I'd rate that as "most highly unlikely"

Ronsonic
03-23-2010, 11:13 AM
Fact is, I think it's fairly miraculous we had new production for as long as we did.

The next number to stock up on is 7591A.

plexi67
03-23-2010, 11:22 AM
The sunn amps that used the 7199 changed to 6AN8s.. their is also info out there on the change. A socket rewire.

mark norwine
03-23-2010, 11:49 AM
The next number to stock up on is 7591A.

NOS? Those are getting rare! Mike? Care to comment?

Happily, though, the JJ is really pretty decent.

Tres Mellow
03-23-2010, 12:44 PM
While we're at it, any preference for either the EH 7591A or the JJ 7591? I have JJ 7591s in my Gemini I and EH 7591As in my Reverberocket 2. I like them both, but the two amps are too different to really compare that way.

mark norwine
03-23-2010, 01:08 PM
When there was nothing else available, the EH "sufficed". "Any port in a storm", as they say.

But I like the JJ's much, much better. When they came out, I latched on & never looked back.

Ronsonic
03-23-2010, 08:26 PM
NOS? Those are getting rare! Mike? Care to comment?

Happily, though, the JJ is really pretty decent.

NOS 7591 have been long gone.

I'm just wondering how long the current production will continue.

Blue Strat
03-23-2010, 09:25 PM
NOS? Those are getting rare! Mike? Care to comment?

Happily, though, the JJ is really pretty decent.


Yep, 7591s are really difficult. Can't remember the last time I had a matched pair in stock.

tejastubes
03-23-2010, 09:34 PM
My expereince with the JJ 7591 is that they are quite bad, and do not sound much like a NOS 7591 at all.

These were used mostly in hifi and most guys have the Ampegs or other amps which use 7591 converted to accept a 6V6GTA instead.

Joshua

Tres Mellow
03-24-2010, 08:36 PM
The JJ 7591s in my Gemini I sound quite good, but I've never had NOS 7591s in it to compare. It came to me with Sovtek 7591XYZs which, as I understand it, are basically 6L6s. Switching to the JJs kept all the low end oomph, but rounded out the tone nicely and made it less hard-edged and sterile for lack of a better word.

As a follow-up to the original post, I reinstalled the RCA 7199 and can't get the problem to reoccur, so I'm inclined to say Mike is right about oxidation in the tube socket being the problem. I guess pulling one 7199 and reseating another was enough to clean up the socket for better contact. I have now cleaned the pins and socket with tuner cleaner. I'm going to clean all the other sockets, too, before I fire it up again. So now I appear to have two good old stock 7199s, so I won't worry about converting to anything else. My Reverberocket 2 uses a combination of a 12ax7 and one part of a 6U10 for the phase inverter, so I won't be needing one there.

donnyjaguar
03-26-2010, 09:15 AM
You can't just swap a 7199 for a 6GH8A with an adapter. The triode sections have different amplification characteristics. If the application is a phase invertor (typical) you need to increase the values of the cathode and plate resistors. If you don't the stage won't be optimum and you'll be drawing too much current through it and the voltage differential between the cathode and plate will be less. This will impact (negatively) dynamic range. The 6U8A is a slightly better choice anyway, IMO.

torrevieja
04-23-2010, 07:01 AM
Hello to everyone. This is my first post here. I'm from Spain so please forget my bad English, I'm doing my best.

I'm a proud owner of a 65' Ampeg Gemini I. My tech restored caps and some broken pot. He changed 12ax7s but 7199, 6CG7 and 7591s are still comming, so I've been playing with these old, tired and original tubes.

Yesterday I pluged in channel two (no fx) and jump, as always, to acordion input of channel 1 but the only sound that came out was from channel 1. I disconnected the jump chord and connected my guitar direct to channel 2 but there's no sound at all. I turned it off, waited a while and then turned on but nothing. However channel 1 works perfectly.

I opened the amp and aparently all connections seem to be in good shape. Could it be caused by the old 7199 or I have to worry?

I'm in love with this amp and I don't want to loose it.

Thanks